(1) Field of the Invention
This invention relates to multiple power source vehicles, and more particularly to improved control techniques for such vehicles.
(2) Brief Description of the Prior Art
Until recently, the major impetus for interest in electric vehicles was due to their potential for reducing vehicular emissions--a major problem in congested urban areas. The realization that our petroleum resources are finite and that the availability of petroleum is already beginning to diminish has renewed interest in electric vehicles as alternatives to the petroleum consuming combustion engine vehicles. Electric vehicles utilize power which can be generated from sources other than petroleum, such as coal or nuclear reactors, thus allowing some of the transportation energy base to be shifted from petroleum to non-petroleum sources. The effectiveness of such a strategy depends upon the acceptance and use of electric cars by the public.
Up to the present time, the major limitations to the acceptability of battery propulsion for automotive applications have been the limited range and low acceleration, poor hill climbing, and low top speed capabilities of such battery powered vehicles resulting from the low energy and low power densities of existing batteries.
The battery for an electric vehicle is heavy and comprises a large fraction of the total system weight. It is known that for an electric vehicle to be safe, the size of the battery should be determined not only by its capability to supply sufficient electrical power for immediate driving requirements, but by its reserve performance capability to supply sufficient electrical power to accelerate or climb hills if required. The reserve power capability of conventional batteries decreases markedly as the battery is discharged. This limits the depth to which a battery can be discharged before reaching the threshold of unsafe vehicle performance capability. This leads to a heavier battery than would be required if deeper discharge could safely be achieved.
It is known, in general, that battery systems are more efficient at low current drains and electric motors are most efficient at certain current loadings.
The prior art contains examples of the use of combustion engine power to supplement a battery powered mover in order to minimize the limitations of battery powered vehicles with respect to battery size, range, and power and to reduce the current drain of the battery. Various of these prior attempts have suffered from the following disadvantages: they have produced a decrease in the energy efficiency of the system, they have led to a decrease in the utilization of off-board generated electricity per trip-mile, they have increased the complexity of the system, and they have led to a sizable increase in the initial cost of the system.
In a method described by Pieper, U.S. Pat. No. 913,846, issued Mar. 2, 1909, an electric motor and a combustion engine are coupled to run at a fixed speed ratio to each other. The vehicle operator commands performance from the electrical system and the combustion engine throttle is controlled automatically so as to keep the battery voltage within prescribed limits. Several drawbacks are attendant to this means of operation. Battery voltage is dependent on state-of-charge, rate of discharge, temperature, as well as other factors, and requires complex computations and controls to compensate for the various factors and so keep the battery at a desired state-of-charge.